Orographic rain

Also called: Orographic rainfall, Relief rainfall, Relief rain · Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Class 7, Ch 3 "Climates of India"; Class 11, Ch 4 "Climate"; Class 11, Ch 10 "Water in the Atmosphere"

Meaning

Orographic rain (also called relief rain) falls when moist wind is forced to rise over a mountain or highland. The rising air cools, its water vapour condenses, and rain falls on the side facing the wind. The name comes from the Greek oros, "mountain".

It matters because most of the world's heaviest rainfall is orographic. In India, it shapes the rainfall map: very wet slopes such as the Western Ghats and Khasi hills sit next to dry rain-shadow lands such as the interior Deccan.

Explanation

How it forms: the windward side

• The basic rule: air must rise before it can rain. Rainfall types are named after how the air is lifted. In orographic rain, a mountain barrier does the lifting.
• The chain on the windward side (the side that faces the wind):
• Saturated, moist wind blows in from the sea and hits a mountain. It is pushed up the slope.
• Higher up, the air pressure is lower, so the rising air expands.
• Air that expands cools. This is adiabatic cooling (cooling caused only by expansion, with no heat leaving the air).
• The air cools to its dew point (the temperature at which air becomes saturated, or full of water vapour).

• The vapour condenses into droplets, clouds form, and heavy rain falls on the windward slopes.

• Clouds: stratus or cumulus clouds form on the windward slopes.

• How long it lasts: the rain keeps falling as long as the moist wind keeps blowing against the mountain.

The leeward side and the rain shadow

• The leeward side is the side sheltered from the wind.
• Why it stays dry:
• The air crossing the mountain has already lost much of its moisture on the windward side.
• As it comes down the leeward slope, it is squeezed (compressed) and becomes warmer.

• Warmer air can hold more vapour, so clouds break up instead of forming.

• Result: a rain-shadow area (the dry land behind a mountain, sheltered from rain-bearing winds). Because the descending air warms, rain-shadow areas are both dry and relatively warm.

Conditions it needs

• Moist wind: usually wind coming off the sea, such as the south-west monsoon from the Arabian Sea or winds from the Bay of Bengal.
• A barrier across the wind's path: a mountain range or highland that stands in the way of the wind.
• Steady wind: the wind must keep blowing towards the slope for the rain to continue.
• Relief that traps air: landforms can increase the effect. At Sohra (Cherrapunji), gorges open towards the south and help trap moist southerly winds [2].
• Height matters even on a smaller scale: Shillong lies in the rain shadow of the Khasi hills, so it gets less rain than Sohra. It still gets more rain than Kolkata. Shillong is high in the hills, so moist air is lifted over it. Kolkata lies on flat plains, where nothing forces the air up.

A caution: real rain is often mixed

• Even at Sohra, the heaviest rain is not purely orographic.
• Hourly records for 1946–1949 show more rain in the early morning than in the afternoon, from April to October [2].
• Why: dry continental air from the east or north-east meets moist sea air from the south-west or south-east. This meeting is a frontal interaction [2].
• India's monsoon mixes all three rain types:
• Orographic: the Western Ghats and the north-eastern hills catch the south-west monsoon.
• Convectional: thunderstorms before the monsoon and during breaks.
• Cyclonic: monsoon depressions, and cyclones after the monsoon.

Where it occurs

India

Barrier	Windward (wet) side	Leeward / rain-shadow (dry) side
Western Ghats (face the south-west monsoon from the Arabian Sea)	Mumbai gets about 200–250 cm a year. Agumbe (Karnataka) and Mahabaleshwar (Maharashtra) get much more	Pune, Bengaluru and the interior of Maharashtra and Karnataka (the Deccan) get much less
Khasi hills, Meghalaya (face Bay of Bengal winds funnelled in from the south)	Sohra (Cherrapunji) and Mawsynram, in East Khasi Hills	Shillong lies in the rain shadow and gets far less
Great Himalaya	Southern slopes and foothills	Leh (Ladakh) gets only about 8.5 cm a year (NCERT, Class 9)

• Sohra (Cherrapunji):
• Location: 25°15′ N, 91°44′ E, at about 1,313 m. It sits on the edge of the southern scarp (steep cliff face) of the Khasi hills, looking over the plains of Sylhet in Bangladesh [2].
• Relief: the town stands on thick sandstone. Cliffs about 600 m high drop into gorges that open towards the south [2].
• Normal rainfall: about 1,080 cm (425 inches) a year. Its highest rainfall in 24 hours was about 925 mm in June 1934 [2].

• World record: the WMO lists 26.47 m (August 1860 to July 1861) at Cherrapunji as the greatest rainfall ever recorded in 12 months [3].

• A sample day: on 20–21 June 2022, Mawsynram recorded 25 cm and Cherrapunji 12 cm of rain in 24 hours [4].

World

• Andes (South America): in the westerly wind belt (about 40°–50° S), the Chilean side (west) is wet. Patagonia (Argentina, east of the Andes) is a dry rain-shadow plateau.
• Sierra Nevada (California, USA): the western slopes facing the Pacific Ocean are wet. The Great Basin (Nevada and Utah, east of the range) is a rain-shadow desert.

Don't confuse with

• Convectional rain: in convectional rain, the ground heats the air and the air rises by itself. In orographic rain, a mountain forces the air up. Convectional rain comes as short, heavy afternoon showers with thunder from cumulonimbus clouds. Examples are equatorial "four o'clock rain" and Nor'westers. Orographic rain lasts as long as the wind blows.
• Cyclonic (frontal) rain: here the air is lifted when warm air rises over cold air at a front, or inside a cyclone. No mountain is needed. Examples are the winter rain of north-western Europe and the east coast of India during cyclones.
• Rain shadow: this is the dry leeward side of the same mountain, not a type of rain. The air there comes down and warms by compression. Pune, Bengaluru, Shillong and Leh are rain-shadow places, not orographic rain sites.
• Shillong vs Kolkata: "rain shadow" does not mean "least rain". Shillong gets less rain than Sohra but more than Kolkata on the flat plains.

Prelims Hooks

• Rain-shadow pairs: Pune and Bengaluru (Western Ghats); Shillong (Khasi hills); Leh (Great Himalaya); Patagonia (Andes); Great Basin (Sierra Nevada).
• Trap: Shillong gets less rain than Sohra but more than Kolkata.
• Sohra (Cherrapunji) is in East Khasi Hills, Meghalaya, at about 1,313 m. It holds the WMO 12-month world record of 26.47 m (August 1860 to July 1861) [2][3].
• Leeward air warms by compression. That is why rain-shadow areas are both dry and relatively warm.
• Leh gets about 8.5 cm a year (NCERT). It is the lowest of the stations in NCERT's Class 9 table, a rain-shadow effect of the Great Himalaya.
• IMD "extremely heavy rain" means more than 204.4 mm in 24 hours. Mawsynram's 25 cm on 20–21 June 2022 was in this class [4].

Mains Points

• Relief controls India's rainfall map (GS-I):
• The windward Western Ghats and Khasi–Garo hills get very heavy rain.
• Interior Maharashtra and Karnataka (Marathwada, north interior Karnataka) lie in the rain shadow.

• These rain-shadow areas are drought-prone. They depend on irrigation and watershed programmes.

• Orographic rain brings its own hazards (GS-III):

• Orographic cloudbursts on the Himalayan and north-eastern slopes cause landslides and flash floods.

• Each region has its own main rain type and its own hazard. Disaster planning in each region must be built around that hazard.

• Reliability and variability of the monsoon:

• Orographic rain on the Western Ghats is reliable.
• Rain from depressions over the plains varies much more.
• This difference shapes cropping patterns and the need for reservoirs.

Related concepts

• Convectional rain
• Cyclonic rain

Read more

• Types of rainfall: convectional, orographic and cyclonic